This invention relates generally to the injection molding of thermoplastic fitments and more particularly to a method for relieving hoop stresses in direct molding of thermoplastic fitments to eliminate cracks formed by the fitments shrinkage about a receiving base upon cooling.
There is an important need for molded fitments in conjunction with certain devices of ordnance, such as proximity fuzes. Two essential requirements proximity fuze front casings must meet to be operationally acceptable are that the fitment be "transparent" to the transmission of electromagnetic radiation signals and that the doomed fitment be aerodynamically shaped. Further constraints placed on the combination of a proximity fuze and fuze cover dome are that the fitment form a water tight seal and that the fitment be capable of engaging interlockingly with the front case assembly of the proximity fuze to withstand extreme rotational shearing forces as the proximity fuze is discharged.
In the past, providing a proximity fuze with a fitment dome was very expensive, both in terms of the basic cost of materials used as well as in terms of the time required to perform the fitment. Previously a Teflon fitment was made separate and apart from the base to which attachment was to be made. A liquid cement, such as Bostik 701, supplied by the B.B. Chemical Co., was applied continuously around the case before the Teflon dome was inserted thereon. A raised lip on the base was then crimped down about the dome with large compressive forces to securingly fasten the dome to the base. This process was not satisfactory as to method or as to machinery required to complete a fitment. Efforts to directly mold a fitment with plastics having similarly been unsatisfactory. After the molding was complete, cracks would still develop as late as perhaps a year after the fitment was formed, due to residual hoop stresses.
In many other diverse applications it is desirable to provide a receiving base with a molded plastic dome. It has been found useful, for example, to place lids on cans, jars, and other containments. In these applications, often neither the shape of the fitment nor the environment the fitment is expected to see is critical.